Sunday, October 18, 2015

Visit to EGO-VIRGO

A meeting of the theoretical and experimental groups of the Italian Institute for Nuclear Physics (INFN) working on the detection of gravitational waves and on theoretical aspects related to them was held at the European Gravitational Observatory (EGO) in Cascina, near Pisa.

The site hosts Virgo, a detector of gravitational waves which -in one year from now- will be ready to join its U.S. cousin Ligo in the decade-long search of gravitational waves from compact objects.

Me standing in front of one of the 3-km long arms of Virgo. Is the motion due to the passage of a gravitational wave? Hope not since the detector is still offline...

What is a gravitational wave?

We answered this question here and here. Gravitational waves are ripples of spacetime produced when massive bodies are accelerated, for example when two neutron stars orbit around each other. They are one of the most striking predictions of Einstein's General Relativity and a direct detection of this effect would be yet another confirmation of this beautiful theory.

How to detect a gravitational wave?
Gravitational waves are just perturbations of spacetime so, in principle, to detect them it's enough to monitor the distance between two objects, or the time between two clocks, or the size of a ruler during the passage of such perturbations, similarly to how the distance between two buoys changes during the passage of a wave. If the distance/time/size changes as predicted by the theory, that's a good hint of the passage of a gravitational wave.

How difficult is to detect a gravitational wave?
It is extremely difficult. As we explained here, the strongest gravitational waves come from th most massive objects moving at nearly the speed of light. Good candidates are supermassive black-hole binaries, binary neutron stars, supernovae etc.. Problem is, these sources are extremely far from Earth (and therefore the amplitude dies off as the inverse of the distance) and also relatively rare in our cosmic neighbourhood. A back-of-the-envelope estimate shows that an object of about 3 km (the size of Virgo's arms, read below) would be deformed just by 0.000000000000000001 meter by the passage of a typical gravitational wave. In practice, a detector should be able to appreciate a change of the order of the size of a proton in a ruler as large as the Earth-Sun distance!

Is this possible?
Believe it or not, it is! The idea of the Ligo and Virgo experiments is to use a sophisticated version of the famous Michelson-Morley interferometer. These experiments use the principles of interferometry to measure tiny differences between two lengths. In very simple terms, these detectors are made of two perpendicular ``arms'' of equal length (typically a few kilometers) with one laser beam each. Thanks to a series of ``mirrors'', the two beams are made to interfere and from the interference pattern one can measure very precisely tiny deviations in the relative size of the arms. Because the arms are perpendicular to each other, a gravitational wave would deform each arm differently and can potentially be detected. Clearly, the real experimental situation is way more complicated than what just described, and reaching the required sensitivity is extremely challenging from a technical point of view. On the top of these technical challenges, almost any disturbance can contribute to the ``noise'' of the apparatus and spoil the detection, including small earthquakes, airplanes flying near the detector, trains, even scientists walking near the apparatus, etc...

One of the 3-km long arms of Virgo in the middle of the Tuscan countryside.

The story of Ligo and Virgo.
The original Virgo and Ligo experiments were operational during the 2000s and didn't detect any signal. This might not come as a surprise given the tiny signal they are hunting. In fact, it's quite challenging to estimate the exact amplitude of the expected signal, because the latter depends on the distance of the source and therefore requires a precise knowledge of the distribution of neutron-star binaries near Earth. On the top of that, each detector is sensitive to gravitational waves whose wavelength is roughly comparable to the size of the arms. This means that not only one needs a close source (for the signal to be strong enough and "stand out" from the detector noise) but also the neutron stars must be just at the right orbital distance in order to emit gravitational waves at the right frequency.  The estimated number of detectable sources is affected by some astrophysical uncertainties, so the original detectors could have made a detection only in ultra-optimistic scenarios.

In the last years both Ligo and Virgo were dismissed in order to upgrade the detectors and to increase their sensitivity by a factor ten. This means that they will be sensitive to a volume of the Universe which is a thousand times bigger than before and the possibility of a detection is much likely. In optimistic scenarios, the detection of a binary inspiral by the second-generation detectors might occur on a monthly basis.

On September 18, 2015, after a 5-year redesign and rebuild, the twin LIGO advanced'interferometers (one in Louisiana and the other in Washington state) officially began their search for gravitational waves. Hopefully, the advanced version of Virgo will join the aLIGO detectors in Fall 2016 (don't blame Italian/French scientists for this delay, Virgo's budget is roughly a factor 10 smaller than Ligo's!). Having more operating detectors is crucial to avoid spurious signals by looking for a simultaneous detection.

Virgo scientists working at the supports of one of the interferometer's mirror

One of the old mirrors of the original Virgo experiment.

What if gravitational waves do not exist after all?
Nobody in the scientific community supports this possibility. The reason is that, although we are still waiting for a direct detection, we have already overwhelming indirect evidence for the existence of gravitational waves. These waves carry energy away from the system. For example, in the inspiral of a neutron-star binary, due to the energy loss, the radial separation between the two stars will decrease with time. Thus, even if we cannot detect gravitational waves directly, we can look at the orbits of compact binaries and see if they shrink according to General Relativity. Hulse and Taylor did precisely this, by monitoring the pulsar PSR 1913+16 over two decades (starting in 1974) while this object was orbiting another neutron star. They found that the orbital period decreased  as predicted by General Relativity to a remarkable precision. These observations pioneered a new era in pulsar astronomy and were awarded a Nobel prize in Physics in 1993. In the decades that followed this discovery it has become an acquired fact that gravitational waves exist and could one day become important astronomical tools to observe our Universe.
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